implemented basic functionality in BookManagementViewModel

- persist/update a TeachingMaterial
- provide a Collection of all TeachingMaterials at startup
updated the provided Collection of TeachingMaterial after having
inserted/updated a TeachingMaterial
bugfix: use Collection instead of Set in interface declaration
removed unused imports
Merge pull request #127 from davherrmann/issue#123

add functionality in BookManagementViewModel